In 1967, 13-cis-retinoic acid was identified in rat tissue extracts and it was postulated to be a natural metabolite of all-trans-retinoic acid..sup.1 The 13-cis-retinoic acid has been found to be at least as effective as all-trans-retinoic acid in promoting vitamin A-dependent growth as well as in controlling epithelial cell differentiation. Recent work suggests that 13-cis-retinoic acid is not solely produced as an artifact of isolation but that all trans-retinoic acid is isomerized in mammals to some extent to 13-cis-retinoic acid..sup.2 The utility of 13-cis-retinoic acid in treating dermatological conditions, its tumor inhibitory properties, and its reduced toxicity relative to all -trans-retinoic acid are known. FNT 1. Zile, M., R. J. Emerick, H. F. DeLuca, "Identification of 13-cis-Retinoic Acid in Tissue Extracts in Its Biological Activity in Rats," Biochim. Biophys. Acta. Vol. 141, pp. 639-641, 1967. FNT 2. Zile, M., R. C. Inhorn, H. F. DeLuca, "Metabolites of All-trans Retinoic Acid in Bile: Identification of All-trans and 13-cis-Retinoyl Glucuronides," J. Biol. Chem., Vol. 257, pp. 3537-3543, 1982.